Ask Mr. Science
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Gravity and Black Holes

Recently (2015) the movie Interstellar came out. Some of the 6th-graders had seen it. For us scientists, what is particularly interesting is that the depiction of the black hole was accurate (as much as possible), guided by the advice and calculations of physicist Kip Thorne.
 artist's rendering of black hole
I started with the concept of escape velocity. As shown in the illustration, you can imagine shooting a cannonball faster and faster, to the point where it would go round and round the earth forever. Some kids were a bit sceptical about this, but I pointed out that here in Santa Fe (where the sky is almost always clear), they can go outside after sunset and in a few minutes they can see a satellite pass overhead, going around and around the earth just like that.

Any spaceship will be bound to the earth in some kind of orbit as long as it's speed is less than 11 km/s (25000 mi/h). Above this escape velocity, the spaceship will fly off into space and never return.

I did the standard demonstration that shows how large masses distort space (actually, spacetime) and attract other masses. I made this large hoop with stretchy cloth. This replaced the thing I used before, a large silkscreen screen.
A marble or 1/2' steel ball will roll in a straight line across this surface without deviation, just as a spaceship, with the engines off and not too close to a star or planet, will float through outer space in a straight line. Of course our model is in 2 dimensions only. Now we hang a massive body like a star in our toy space. The most appropriate object I had on hand was a potato-sized smooth river rock. When placed on the nylon, it depresses the surface 1/2' or so right under the rock, flattening out again towards the edge of the frame. Now, when the marble is placed on the screen, it will be attracted to the rock. Also, if you roll the marble on a trajectory close by the rock, its path will be noticeably deflected. With some practice, you can even get your marble to go into orbit around the river rock.
If you roll the marble slowly in the vicinity of the central rock, it will roll back to the center. If you roll it too fast, it will escape. A black hole is simply any object where the gravity is so strong that the escape velocity is larger than the cosmic speed limit, the speed of light.

After the observation by LIGO of merging black holes, I added ... (more)

1998, updated 2018


Electricity and magnetism

I spent two weekly hours on the subject. First, I wanted to show that most of the electricity around us comes from 2 different sources: batteries and generators.
For the batteries, I did the standard stuff: any acid plus any two metals make a battery. I had an orange, a lemon, and some scraps of cloth with vinegar. For different metals, I had thick copper wire, big galvanized nails (for a zinc surface), steel nails, aluminum foil, pennies. Then, with some telephone wire and my old analog voltmeter, we made several batteries: the standard one where you stick nails and wire of different metals into the lemon or orange. We noted the different voltages, and hooked them up in series to get a little higher. Unfortunately, one or two lemons is not enough to light up a flashlight bulb. We made a 'Volta pile' (reference to the original pile here) by stacking aluminum foil, vinegar-soaked cloth and pennies.
For the generator part, I had made a simple generator:

simple generator
  • In a board, I screwed four 3" drywall screws in a 1.25x2" rectangular pattern. Around this I wound a bunch of telephone wire.
  • I had a pretty strong fridge magnet, about 2x1x3/8". I fixed an axle to it by bending a piece of coathanger wire, and tying it in place with string. The wire stuck out about 2" on either side.
  • Two pieces of cardboard, folded, glued to the wood base, and with slots in the top to hole the magnet, formed pretty good bearings.
  • Slip the magnet into place, wrap masking tape around the axle right next to the cardboard, so that the axle cannot slip sideways. The masking tape also make a fatter place for your fingers to grab onto when you twirl the magnet.
  • Of course, the dimensions given here all depended on the particular magnet you happen to have around. The goal is to have the wire as close to the twirling magnet as possible, while at the same time have enough clearance so that the magnet can twirl freely.

Twirling the magnet produces enough voltage to register easily on the voltmeter (as AC, not DC, but I glossed over that detail), but again not enough for a flashlight bulb. [Would an LED work?]

een voorvork, voorwiel, dynamo en achterlichtje
Here's a closeup
The next week I realized that I did have a generator that would light up a lamp. Being a Dutchman, I had in my box of bike parts an ordinary 'dynamo', or bike generator. This thing mounts on the front fork of your bike, and when engaged, presses against the tire. Out of the same box came a front bike fork, and to complete the demonstration, I borrowed a wheel from my garden cart, which I had built using 2 front bicycle wheels. I could mount the whole assembly upside down with some scraps of wood, and light up a bike light brightly with even a gentle turn of the wheel.
In the real world, what turns generators? We talked about dams, and generators powered by oil, gas, coal and nuclear power. The children were clever enough to add solar and wind power to the list. They were well aware of the fact that fossil fuels cause plenty of environmental problems, but not that these fuels were going to run out in their lifetimes, and that they had a big problem to solve when they grew up to be engineers.

31 March, 7 April 98, 2 and 9 Feb 2000


What is the Aurora?

Earlier in the year, we had looked at the sun with the sunspotter

Time to talk about the sun and all that. Most of what I talked about is on the first two web sites on the list below: How the sun is not as quiet and peaceful as it may seem, that great blasts of material are blowing away from the surface all the time, and that this solar wind roars by the earth. How the Earth is like a big bar magnet that channels all that stuff to the North and South poles. That the space shuttle typically flies lower than the Aurora. One of the harder concepts (for these 3rd-graders) was that the atmosphere does not just stop at a well-defined altitude, but that there is (very) thin gas up where the shuttle flies, and higher still.
Check out the top 2 links below!

To show that the sun is not without blemishes, I also had brought in my home-built sunspot viewer, and we took it outside and observed a fair number (5 or so) sunspots that day. More on the sunspotter below.

I brought a demo that showed the earth's magnetic field: a bar magnet rests under my screenprinting screen, white sheet on top, and a cutout of the earth. Sprinkle iron filings around, and the shape of the magnetic field appears.

  • Screen and paper cover
  • Magnets and iron filings
  • Earth cutouts
  • Compass
  • Funnel, brush for cleanup      


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